New Exo-Functional Cyclophane Hosts. Synthesis and Crystal Structures of Inclusion Compounds

The new cyclophane type host compounds 1–3, containing rigid aromatic units and two exo-topic carboxylic acid functions, have been synthesized. Crystalline solvent inclusions, involving the dicarboxylic acid hosts and their corresponding ester intermediates 6 (a–c), namely 1·DMSO (1:4), 2·pyridine (1:3), 3·pyridine (1:4), 6a·pyridine (1:2) and 6a·benzene (1:2), have been prepared and studied using X-ray diffraction on single-crystals. Moreover, X-ray structure analyses of the solvent-free crystals of the 6 (a, b) intermediates were also carried out for comparison. Co-crystals of the carboxylic hosts 1–3 contained H-bonded 1:2 host-guest associates as building blocks, together with additional space-filling guests, whereas only loosely bounded space-filling solvent molecules were found in the two solid inclusion compounds of the 6a cyclophane ester host. In addition to the mentioned conventional H-bond interactions between carboxylic hosts and their guests, the crystal structures proved to be held together by relatively weak C–H…O bonds besides the ordinary van der Waals' interactions. Packing relations, and the effects of structural variations, guest molecules and anisotropic packing forces on the conformation of the semi-rigid cylcophane ring have been discussed and compared in seven crystal structures.